Cheney's algorithm: Difference between revisions

Revision as of 13:11, 29 December 2015

Revision as of 12:25, 12 March 2016

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{{Use dmy dates|date=August 2012}}

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'''Cheney's algorithm''', first described in a 1970 [[Association for Computing Machinery|ACM]] paper by C.J. Cheney, is a method of [[garbage collection (computer science)|garbage collection]] in computer software systems. In this scheme, the [[heapMemory (datamanagement#Dynamic memory structure)allocation|heap]] is divided into two equal halves, only one of which is in use at any one time. Garbage collection is performed by copying live objects from one semispace (the from-space) to the other (the to-space), which then becomes the new heap. The entire old heap is then discarded in one piece.

Cheney's algorithm reclaims items as follows:

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Once all to-space references have been examined and updated, garbage collection is complete.

The algorithm needs no stack and only two pointers outside of the from-space and to-space: a pointer to the beginning of free space in the to-space, and a pointer to the next word in to-space that needs to be examined. For this reason, it's sometimes called a "two-finger" collector --- it only needs "two fingers" pointing into the to-space to keep track of its state. The data between the two fingers represents work remaining for it to do.

The forwarding pointer (sometimes called a "broken heart") is used only during the garbage collection process:; Whenwhen a reference to an object already in to-space (thus having a forwarding pointer in from-space) is found, the reference can be updated quickly simply by updating its pointer to match the forwarding pointer.

Because the strategy is to exhaust all live references, and then all references in referenced objects, this is known as a ''breadth-first'' list copying garbage collection scheme.

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== Equivalence to tri-color abstraction ==

Cheney's algorithm is an example of a [[Tracing garbage collection#Tri-color marking|tri-color marking]] garbage collector. The first member of the gray set is the stack itself. Objects referenced on the stack are copied into the to-space, which contains members of the black and gray sets.

The algorithm moves any white objects (equivalent to objects in the from-space without forwarding pointers) to the gray set by copying them to the to-space. Objects that are between the scanning pointer and the free-space pointer on the to-space area are members of the gray set still to be scanned. Objects below the scanning pointer belong to the black set. Objects are moved to the black set by simply moving the scanning pointer over them.

When the scanning pointer reaches the free-space pointer, the gray set is empty, and the algorithm ends.